Followup to r1449023 (which removed GSSAPI from configure), remove the
remnants of GSSAPI.

* aclocal.m4: Remove include.

* Makefile.in: Remove variable.

* build.conf:
  (gssapi): Remove section. 
  (libsvn_ra_serf): Adjust link.

* svn_error_codes.h
  (SVN_ERR_RA_SERF_GSSAPI_INITIALISATION_FAILED): Deprecate.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1455183 13f79535-47bb-0310-9956-ffa450edef68